1. Field of the Invention
The present invention generally relates to a medical apparatus, and more particularly to a permanent magnet used for magnetic resonance imaging for medical diagnosis.
2. Brief Description of the Prior Art
The magnetic resonance imaging apparatus is a medical apparatus developed in 1980's, and the most advanced clinical diagnosis apparatus for diagnosing pre-cancer and other diseases. The magnet is a key part of the magnetic resonance imaging apparatus, the SNR (signal-to-noise ratio) of the magnetic resonance imaging apparatus has a direct relation to the performance of the magnet, so that performance of the magnet determines the quality of the image to a certain extent. The permanent magnet has a simple structure without additional means and is convenient to maintain, can be operated safely, and the area of the stray magnetic field there around is small. Therefore, the scientific, technological and medical professionals pay more and more regard to the magnetic resonance imaging apparatus using a permanent magnet.
However, the conventional box-shape permanent magnet formed of ferrite magnetic material is low in magnetic intensity, heavy, large in volume, closed and poor in practicability. In the early stage, weight of a magnet of 3000 Gs was substantially 100 tons, so that it was difficult to transport and mount the magnet. Subsequently, a new magnetic material, i.e. NdFeB, was developed, and many countries have invested a lot of work and money in research and development of the magnetic resonance permanent magnet having low field strength. Recently, there has been proposed a magnet of so-called open type, which has high magnetic energy product, good uniformity, and small volume. This type of magnet has the following advantages: a patient can lie on a half-open examining table, thus eliminating the patient's dread occurring in the conventional MRI examination, so that it is easily acceptable for children or other apprehensive patients. In addition, during the MRI examination, the patient is not required to lie on or lie face down on the examining table, and may be seated on the table so that they may be examined on a part of the body thereof such as the extremities. In addition, it is possible to provide interventional therapy to the patient. Therefore, this type of magnet has developed rapidly and a lot of products are available on the market. Since this type of magnet is cheap, the market share thereof has increased gradually. However, the above conventional products have a separated structure, the yoke part of the magnet is formed by a plurality of portions assembled to each other. Therefore, it is difficult to ensure parallelism of the upper yoke to the lower yoke during manufacturing and assembling, thus deteriorating the performance of the magnet to a certain extent, more particularly, decreasing the mechanical strength of the magnet and affecting the uniformity of the magnetic circuit.